Heterogeneous photochemistry of gaseous NO2 on solid fluoranthene films: A source of gaseous nitrous acid (HONO) in the urban environment

Using a coated flow tube equipped with several near-ultraviolet emitting lamps, (range 300–420 nm), we examined the effect of actinic radiation on the heterogeneous loss kinetics of gaseous nitrogen dioxide on solid Polycyclic Aromatic Hydrocarbon (PAH) films deposited on a Pyrex substrate. The PAH studied was mainly fluoranthene, with additional tests on phenanthrene and pyrene. No dependence of the uptake coefficient (γ) was observed either with temperature, or with relative humidity. In the dark, the reaction was very slow but was greatly enhanced by increasing the UV-A light intensity. A linear dependency of the reaction kinetics with the photonic flux was observed. Under atmospherically-relevant NO2 concentrations (20 ppbv), the uptake coefficient was about 1 × 10−6. The uptake coefficient variation as a function of the NO2 concentration suggests a Langmuir–Hinshelwood (L–H) type mechanism. This is characterized by the adsorption of NO2 on the solid surface followed by a chemical reaction. The corresponding equilibrium constant (K′) and the surface reaction rate constant (ks1) were found to be 3 × 10−2 ppbv−1 and 5 × 10−5 s−1 respectively for the photo-enhanced uptake of NO2 on the fluoranthene substrate. Particular attention was given to the detection of the gas-phase products showing the photo-enhanced reduction of NO2 to HONO and NO via a photosensitized reaction involving excited states of the PAH. Additionally we investigated the reactivity of PAH in the presence of nitrates in order to better understand if HONO generation mechanism could be explained by a first deposition of nitrates (generated via NO2 hydrolysis) on the solid surface.